Electrochromic materials, that is, materials that change color oxidation or reduction, have become of increased commercial importance. Electrochromic devices are electrochemical cells that comprise an anode, a cathode, an electrolyte, and one or more electrochromic materials that are either surface confined or in solution, and provide a means to change light absorption properties of the device such that a color change is observable as a result of the electrochemical reaction at one (or both) electrode(s) (anode and cathode).
A wide variety of electrochromic materials and devices have been described. Electrochromic devices have found use as smart windows, automatically dimmable mirrors, and as static or modulated displays. Reviews on the various categories of electrochromophoric materials have been published. (See for example, N. Rowley and R. Mortimer, “New Electrochromic Materials”, Science Progress (2002), 85 (3), 243-262 and “Electrochromic Materials”, in Proceedings of the Electrochemical Society, K. Ho, C. Greenberg, D. MacArthur, eds., Volume 96-23, 1997). The most important classes of compounds which demonstrate the electrochromic effect are: transition metal oxides, prussian blue systems, viologens, conducting polymers, and transition metal and lanthanide coordination complexes.
Electrochromic systems employing the metal oxides as electrophores exploit the color change of a layer of transition metal oxide, for example, WO3, NiO, MoO3, IrO2 or Co2O3 deposited on a transparent conductive electrode. A closely related device utilizes prussian blue (inorganic “polymeric” materials based on iron ferricyanides such as FeIIFeIII(CN)6− and analogs) as the electrochromic material. Because the color change in systems that employ these inorganic electrochromic materials is a result of the intercalation of counterions from the electrolyte into the bulk of the inorganic layer during oxidation or reduction, the associated color changes in such devices are rather slow, on the order of tens of seconds even for relatively small area devices. Response time may be improved by making the metal oxide layer very thin, but this leads to low contrast devices that are unacceptable. In addition to having relatively slow response times, these materials also exhibit broad absorption bands. Thus, these inorganic electrochromic systems are presently limited in their application to electrochromic window applications.
Electrochromic polymers have been widely studied and can offer a range of color changes during oxidation or reduction. Most prevalent have been polymers formed by the chemical or electrochemical oxidation of aromatic compounds such as pyrolle, thiophene, aniline, furan, carbazole, azulene, and indole. Electrochromic devices have been described wherein the electrochromic polymer has been deposited as a film on a transparent conducting substrate. Many of the polymeric electrochromophores have limited stability in the oxidized or reduced states. Limitations due to slow response time during operation of the devices is also encountered as counterions must diffuse into and out of the polymeric layer in order to maintain charge balance.
Transition metal complexes and phthalocyanines have also been explored for use as electrochromic materials but have found limited application.
Molecular electrochromic materials that change color upon oxidation or reduction have recently received much attention. By far the most utilized group of electrochromic materials are the 1,1′-disubstituted-4,4′-bipyridinium salts, better known as viologens. The electrochromophore molecules may be dissolved in an electrolyte solution sandwiched between two opposing transparent electrodes. Such devices comprising at least one soluble molecular electrochromophore and a second soluble (molecular) or insoluble (metal oxide or prussian blue) electrochromophore have been described in U.S. Pat. Nos. 6,433,914, 4,902,108, and 5,128,700. A variation on this design utilizes electrochromophore molecules that are bound to, or otherwise incorporated into, a polymeric film deposited on a transparent conducting electrode, as disclosed in A. Sammells et. al., J. Electrochem. Soc., 1986, vol. 133, 1270; “Electrochromic Materials”, Ho and Greenberg, editors, 1997, The Electrochemical Society, Inc., Pennington, N.J. Alternatively, electrochromophore molecules can be directly attached either by a chemisorption or a chemical bond forming method to the surface of a transparent, nanocrystalline film electrode. The attachment of a monolayer of electrochromophore molecules to transparent, high surface area, nanoporous-nanocrystalline semiconducting electrodes can yield devices with high electrochromic contrast changes and relatively short switching times. Such nanocrystalline electrochromic devices are described in Displays (1999) 20, pp. 137-144, Fitzmaurice et. al, J. Phys. Chem. B 2000, vol. 104, 11449-1459, WO-A-97/35227, WO-A-98/35267, U.S. Pat. Nos. 6,301,038, 6,605,239, WO127690, WO 98/3267, and U.S. Pat. Application No. 2002/0181068, U.S. Pat. Application No. 2002/0021482A1 (comprises at least one electrode incorporating a semiconducting nanostructured metal oxide film modified by chemisorption of a molecular electrochromophore).
By virtue of the nanoporous nature of the nanocrystalline metal oxide films, the monolayer of chemisorbed electrochromic molecules are in contact with both the electrode and the electrolyte solution containing the counterions, and thus facilitate fast charge-compensation during the electrochromic redox processes and ensure rapid switching time. The nanoporous-nanocrystalline metal oxide that has been shown to possess good film forming qualities and have the high conductivity and transparency is TiO2 as disclosed in Gratzel et. al. in J. Amer. Chem. Soc., 1993, vol 115, 6382. Nanocrystalline SnO2 has also been described for use in electrochromic devices in Fitzmaurice et. al, J. Phys. Chem. B 2000, vol. 104, 11449-1459.
In addition, for use in combination with transparent nanocrystalline metal oxide electrodes, such as TiO2, it is necessary for the electrochromophore to have a redox potential that lies close to the conduction band edge of TiO2 at the liquid-electrolyte interface in order to reversibly transfer electrons from the conduction band to the molecular electrochromophore. Thus it is necessary for the molecule to undergo a color change upon electroreduction when used with TiO2.
Molecular electrochromophores that have been shown to undergo a reversible color change from a transparent, or lightly colored, state to a stable, intensely colored, (radical) state upon electroreduction include viologens, related diazapyreniums, perylenedicarboximides, and napthtalenedicarboximides. Molecular electrochromophores that have been shown to undergo a color change from a transparent, or lightly colored, state to a stable, intensely colored, (radical) state upon electrooxidation include phenothiazines, triarylamines, substituted phenylendiamines, and ferrocene, among others.
The viologens are the most studied and commercially utilized molecular electrochromophore materials. The dicationic form, that is, the oxidized form, is colorless and can be electroreduced to produce a stable, intensely colored radical cation. Suitable choice of nitrogen substituents to attain the appropriate molecular orbital energy levels can, in principle, allow color choice of the radical cation. Viologens containing short alkyl chains yield blue-purple colored radical cations. Those containing longer chains or aromatic-ring containing substituents tend to give crimson colored radicals due to charge-transfer complexation (dimerization) of various viologen states. The typical N,N′-dialkylviologen cation radical exhibits a maximum absorption around 600 nm with an extinction coefficient between 10,000 and 20,000 M−1cm−1 in an organic solvent. The viologens have, in general, low electrochemical reduction potentials (for example between −0.1 and −0.5 V vs SCE) and thus well suited for use with nanocrystalline TiO2 materials. Although viologen electrochromophores give stable and intense colored radical states, the viologens are disadvantaged in that the variety of synthetically allowable substitutions and range of colors attainable is quite limited. Furthermore, the soluble forms of viologens, in particular, N,N′-dimethylviologen, are known carcinogens and thus pose serious health concerns regarding the manufacture and disposal of electrochromic devices containing such materials.
Other electrochromophores, for example those from the perylenedicarboximide and naphthalenedicarboximide class may have low electrochemical reduction potentials, however they are difficult to synthesize and are difficult to utilize because of very limited solubility in common solvents.
Although a vast number of organic molecules, particularly in conjugated carbocyclic and heterocyclic aromatic compounds, undergo color change upon electrochemical oxidation or reduction, there are only a limited number of such molecules that have found high interest or practical use in electrochromophoric systems. Desirable features for a molecular electrochromophore include a high degree of transparency in the visible color region in the “off” state (non-reduced or non-oxidized states, high absorption in visible spectral region upon electroreduction or electrooxidation (“on” state), low electrochemical potential for reduction/oxidation, high stability in the “on” or “off” state (bi-stable), color tunability by synthetic variation of the parent molecular structure, convenient synthesis, high solubility in common solvents, and low toxicity. Preferred electrochromophores are those that undergo the highest contrast change upon oxidation or reduction, that is, from a colorless to a highly colored state, or from a colored state to a colorless one. Less desirable are electrochromophores that change from one colored state to different colored state upon oxidation or reduction.